Doppler-weather signal processing for a rapid RF- and PRF-agile airport surveillance radar

The capabilities of surveillance radar assets are being extended to provide Doppler-weather sensing at select major airports. Currently, the ASR-9 radar can be augmented with a wind-shear processor (WSP) to provide data on wind shear, gust fronts, storm tracks and local precipitation levels. The radar, however, is primarily an aircraft surveillance system that features several characteristics that pose a challenge to Doppler-weather processing. These include a fan-beam antenna pattern in the vertical dimension and a relatively short coherent pulse burst that is punctuated with interim shifts in the pulse repetition frequency (PRF). As a result, weather spatial resolution, wind measurement accuracy and surface-clutter suppression all suffer to a degree, relative to dedicated weather radars. The WSP incorporates signal processing that somewhat mitigates these obstacles. By comparison, the ASR-11 radar possesses most of the same limitations for Doppler-weather processing, and more. As a lower-cost solution for aircraft surveillance at secondary airports, the radar features some 17 dB less peak power than the ASR-9. To achieve the required performance in detecting aircraft, the ASR-11 uses a longer pulsewidth and much more rapid switching of the RF and PRF than the ASR-9. In addition, the average PRF of the ASR-11 is substantially lower than the average of the ASR-9. All of these differences make the ASR-11 less well suited for Doppler-weather processing than the ASR-9. These differences also mandate a new approach to processing the weather return. This paper describes a signal processing design that is adapted to the ASR-11 waveform. The key elements of the design are an adaptive whitening filter for separating multiple weather sources within the fan beam, a dealiasing logic that incorporates the four ASR-11 PRFs to resolve wind speeds, and a measurement acceptance logic that suppresses erroneous windspeed indications.